1. Field of the Invention
This invention relates in general to antenna systems, and in particular to a gridded reflector antenna system.
2. Description of Related Art
Communications satellites have become commonplace for use in many types of communications services, e.g., data transfer, voice communications, television spot beam coverage, and other data transfer applications. As such, satellites must provide signals to various geographic locations on the Earth""s surface. As such, typical satellites use customized antenna designs to provide signal coverage for a particular country or geographic area.
In order to provide good cross-polarization performance over the geographic region of interest, a shaped dual reflector geometry is often used. The subreflector and/or main reflector is shaped to generate a beam pattern that covers the intended coverage geographic region.
An advantage of dual reflector designs is that the main reflector is thin and therefore generally easy to package and stow in the confines of the launch vehicle volume constraints. A typical dual reflector antenna system can provide one beam for each of two linear polarizations. However, typical dual reflector antenna systems have a main reflector that has only one solid surface, and therefore can generate only one distinct beam shape.
Alternately, a xe2x80x9cdual-griddedxe2x80x9d shaped reflector system may be used to produce beams over the desired coverage area. This type of antenna system is a shared aperture system having two separate reflective surfaces, one reflective surface for each polarization. Each reflective surface, also called grids, maybe shaped to produce a distinct beam shape for each polarization.
The related art shapes the grid pattern surface geometry, e.g., places undulating waves and/or distorts the grid surface in the z-direction to shape the beam to the desired size and/or location. Further, the related art moves the feed horn location to again move the beam location or change the beam size. The related art requires for a single reflector with two feed horns of opposite polarizations, the focal points of each grid must be separated to provide adequate cross-polarization performance. The resulting reflector shell becomes large and thick, and therefore difficult to package and stow within the confines of the launch vehicle constraints. The use of multiple antennas can also produce multiple beam patterns, however, multiple antennas within a system also produce space and deployment problems for the satellite and make it difficult to design the satellite to fit within the launch vehicle volume constraints. Further, each satellite must have a custom designed feed horn location and/or a custom shaped reflector to enable the satellite to deliver the desired beam pattern and locations.
It can be seen, then, that there is a need in the art for antenna reflectors that provide multiple distinctly shaped beams. It can also be seen that there is a need in the art for antenna systems that provide distinctly shaped beams for multiple polarizations that are easy to stow within launch vehicle constraints. It can also be seen that there is a need in the art for antenna systems that can deliver a desired beam pattern and location without having to custom design each reflector geometry, e.g., nominal focal axis of the reflector, and feed horn location.
To overcome the limitations in the prior art described above, and to overcome other limitations that will become apparent upon reading and understanding the present specification, the present invention discloses a method for broadcasting, a signal, and an antenna system. The antenna system comprises a feed horn and a reflector. The feed horn provides a radio frequency (RF) signal. The reflector is aligned with the feed horn and is illuminated by the feed horn, and comprises a reflective grid. The reflective grid lines are substantially parallel as viewed from a geographic location of a desired output beam from the antenna system.
A method in accordance with the present invention comprises illuminating a reflector with RF energy emanating from a feed horn, the feed horn being substantially located at a focal point of the reflector, wherein the reflector comprises a reflective grid, and reflecting the RF energy with the reflective grid, wherein lines of the reflective grid are substantially parallel as viewed from a geographic location of a desired output beam from the antenna system.
The present invention provides an antenna system that provides distinctly shaped beams that are easy to stow within launch vehicle constraints. The present invention also provides an antenna system that provides distinctly shaped beams for multiple polarizations that are easy to stow within launch vehicle constraints. The present invention also provides antenna systems that can deliver a desired beam pattern and location without having to custom design each reflector reflector geometry, e.g., nominal focal axis of the reflector, and feed horn location.